To form a wiring substrate with metal particles, generally used is a method of applying metal paste, which is an adhesive or paste containing metal particles, to a substrate through coating or printing.
Among various kinds of metal paste, the most common example is one containing silver (Ag) particles, which is used for electrical connection between terminals, or mending of disconnection. Such a metal paste normally contains metal fine particles in size of several microns to several ten microns, and it causes electrical coupling by using contact force between the respective particles (intermolecular force). As well as the metal particles, general metal paste contains a corrosion-resistant material, an adhesive material such as an epoxy, or solvents of this material etc.
Further, in recent years, finer metal particles with diameters of several nanometers to several tens nanometers (hereinafter referred to as ultrafine particles) has been attracting much attention. With the small diameters, the ultrafine particles offer such advantages that they are in touch with an object with more contact points between themselves and the object; they can constitute a thinner film; and they ensure superior flatness of the surface of substrate, etc.
Further, for the ultrafine particles around 100 nanometers or smaller, their characteristics, such as a decrease of melting point or an increase of reactivity (which are called size effect) becomes more considerable. Accordingly, a decrease of size causes a significant increase of surface energy with respect to internal energy.
When such ultrafine particles with high surface energy are in contact with each other, they are easily coalesced together, and the coalescing state is maintained until the size effect wears off. Thus, a metal thin film made up of ultrafine particles coalesced to each other offer a low resistance value, more closer to a bulk, compared to a material in which metal particles are not coalesced but aligned, and electrical conduction was made only by contact force between the particles.
The ultrafine metal particles with the described characteristics have already been used as a wiring material. One example is a conductive paste containing both large metal particles (with a diameter of μms) and ultrafine metal particles (with a diameter of nms). In this conductive paste, the ultrafine particles fill the respective gaps between the large metal particles, thus offering a low resistance.
Further, another example of a wiring material made of ultrafine metal particles is a colloid material in which the ultrafine particles coated with an organic material are dispersed in a solvent. The colloid material allows easy formation of a metal thin film by being applied to the object and then by being subjected to baking. In this manner, the organic material coating the ultrafine particles are evaporated through baking, and therefore the ultrafine metal particles are easily coalesced, thus easily forming a metal thin film with a resistance close to a bulk.
Incidentally, because of their small diameters, the ultrafine metal particles are energetically active, and their reactivity relies on the activity. Therefore, when the ultrafine metal particles are applied to the object having a metal surface so as to form a film, the metal surface and the ultrafine metal particles are coupled with each other. Thus, the created film will not remove from the metal surface.
However, when the object to which a film is formed by being coated with ultra fine particles is an insulative material such as an oxide, the ultrafine metal particles are only placed on the insulative material unlike the case of sputtering or vapor deposition in which the particles are injected into the object by some energy. Therefore, particularly for the ultrafine metal particles having low reactivity with respect to other materials such as a noble metal, the applied particles are more easily removed from the surface, thus immediately failing “a peel test” using a tape.
In view of this problem, Document 1 (Japanese Laid-Open Patent Application Tokukaihei 03-263391/1991 (published on Nov. 22, 1991) discloses a method (first method) using a wiring material in which ultrafine metal particles contain several % by weight of glass frit. Also Document 2 (Japanese Laid-Open Patent Application Tokukaihei 03-179794/1991 (published on Aug. 5, 1991) discloses a method (second method) of applying an epoxy adhesive, which is either cured or half-cured, on a surface coated with an insulative material, and applying ultrafine metal particles on the surface, which is then pressed and heated. Further, FIG. 8 shows still another method disclosed in Document 3 (Japanese Laid-Open Patent Application Tokukaihei 11-80618/1999 (published on Mar. 26, 1999), in which a substrate 101 coated with ultrafine metal particles 103 is baked, and then further coated with silica material 102, which is cured after application.
Incidentally, the wiring material containing glass frit, such as the one used in the first method, ensures adhesion between the wiring and the insulative material. However, since the glass frit operates as a resistance component in the wiring, the wiring should contain as small amount of glass frit as possible, so as to obtain a low resistance close to the wiring formed by a metal (such as Al) through sputtering, vapor deposition or the like.
However, when the wiring contains less amount of glass frit, there arise a problem of removal of the wiring from the substrate.
Therefore, the first method cannot ensure both of the adhesion between the wiring and the substrate, and the low resistance of the substrate.
Further, in the second method of applying a cured or half-cured epoxy adhesive on a surface coated with an insulative material, and applying ultrafine metal particles on the surface, which is then pressed and heated; the pressing may cause deformation of the wiring material in case of forming a thin film, thus arising a risk of breakage of wires.
Further, in the second method, the adhesive is used to ensure adhesion with the substrate, and therefore some kinds of metal particles can remove from the adhesive. Accordingly, the second method requires ultrafine metal particles with superior adhesion with respect to the adhesive. To meet this requirement, the material for wiring has to be selected from a smaller range.
Further, in the third method of applying and curing a silica material on ultrafine metal particles applied on a substrate; the silica material only fixes the wiring to the insulative material to prevent the wiring from being removed from the insulative material, but it does nothing to improve adhesion between the wiring and the insulative material. Thus, there is still a risk of removal of the wiring depending on the occupied area of the film of ultrafine metal particles formed on the substrate, if the adherence between the metal ultrafine particles and the silica material are not ensured.
Further, in the third method, adhesion between the wiring and the substrate is ensured with permeation of the silica material through the gaps between the ultrafine metal particles. Thus, the adherence can be ensured when the film is thin enough to provide sufficient gaps between the ultrafine particles; however, when the ultrafine metal particles are coalesced with each other to become a continuous film, and the silica material can no longer permeated through the gaps between the particles. Thus, the third method cannot ensure improvement of adhesion between the wiring and the insulative material either. Further, through such a processing, the silica covers the surface of the film, thus causing some difficulties for making electrical contact from the surface.